The materials that are used in devices to control the flow of hot gases are subject to special mechanical, thermal, and often chemical requirements. For example, the control of a solid propellant missile is achieved by regulating and directing the flow of hot exhaust gases produced by combustion of the propellant, and this flow control is accomplished by means of valves, ducts, nozzles and the like that are continuously exposed to these hot gases. The exposed surfaces of the flow-controlling means must be comprised of materials that are compatible with the flow velocity and gas temperature, and also with the chemical composition of the exhaust gas. In this particular application, the material density is also a matter of concern because of the desirability of lightness in the weight of the missile.
Carbon based materials are often useful for this application. In some cases, graphite and carbon composites have been found to be preferred materials. They have the desirable feature of low density, and they maintain their strength relatively well at high temperatures. However, these materials also have the drawback of low hardness, so that they tend to erode in the presence of high-velocity gas flow. Furthermore, these materials are susceptible to chemical attack by certain compositions of the flowing gases at high temperatures.
Therefore, the exposed surfaces of flow-handling devices composed of graphite, or other carbon based materials, are generally treated to overcome these drawbacks. The treatment usually consists of converting the exposed surface to a layer of ceramic material to improve the resistance to abrasion, particulate impact and chemical attack, as well as the surface strength, under the exposure to flowing high temperature corrosive gases. Various elements and compounds have been used to form these surface layers, including oxides of boron, phosphorus, silicon, aluminum, and chromium, silicides and silicon based coatings, and carbides of silicon and various metals.
Coated carbon-based surfaces in some cases suffer from a further drawback caused by the thermal expansion mismatch between a coating and the substrate. When the surface is exposed to high temperature gas flow, this mismatch can result in cracks in the coating and degradation of adhesion between the coating and the substrate. This problem is especially serious for graphite and other carbon-carbon composites because of their very low coefficient of thermal expansion. Multilayer coatings have been developed in which the composition of the layers is varied so that the coefficient of thermal expansion is graded from the substrate to the outermost layer. This technique only partially improves the performance of carbon-carbon composite materials.
In addition to carbon-carbon composites, materials have been fabricated of metal-matrix and ceramic-ceramic constituents and found to exhibit the desirable properties of low density and superior performance under high temperature conditions. A further technology for fabricating materials with these properties is the use of refractory foams. This technology is described in the article by Richard B. Kaplan and Robert H. Tuffias entitled "High Temperature Composite Structures Are Fabricated With Deposition Process," published in Research & Development, February 1989, pages 118-120. This article describes materials comprised of porous carbon foam with a coating of refractory material deposited on the internal pore surfaces. Examples of refractory materials that can be used include niobium, tantalum, rhenium, tungsten, and the platinum group metals. In addition, the oxides, nitrides, carbides, and borides of these metals are suitable, as well as those of hafnium, zirconium, titanium and silicon. The external surfaces of the foam may be closed out or "skinned" by applying a nonporous sheet of adherent material to these surfaces. These refractory foam materials may be manufactured to exhibit improved mechanical and thermal properties, and the density of the material may be varied within a certain range, depending on the constituents of the material.
In all of the foregoing techniques, the most common method of applying a surface coating is by chemical vapor deposition (CVD). For refractory foam materials, a variant of this method is used, known as chemical vapor infiltration (CVI). The CVD technique is a well-known method that is used for surface treatment and fabrication of layered materials in many applications other than the manufacturing of control systems for high temperature gas flow. However, it has the drawback of being inherently expensive and time consuming. It is often difficult, if not impossible, to use this method for surfaces having a complex geometry such as the internal surfaces of valves or gas flow passages. Furthermore the buildup of coating on a surface by CVD may produce dimensional changes in a given physical part. Such changes must be taken into account in designing parts that are intended to fit or mesh together, such as screw threads or pistons and cylinder bores.
Another method for surface treatment is the slurry coating technique. Less development has been done on this method than on the CVD method. However, the slurry coating technique has the ability to provide contiguous coatings on rough surfaces, and the method requires no major equipment expenditures. This method consists basically of mixing powders of desired elements with a carrier fluid to form a slurry, having a viscosity that is sufficient to allow the slurry to be painted, sprayed or otherwise coated on the surface that is being treated. The surface is coated with the slurry by painting, spraying, dipping or similar methods, and the slurry adheres to the surface. The surface is then heated to a temperature such that the carrier fluid evaporates and the powder particulates react chemically, both with each other and with the substrate. The reactions cause the desired coating to form on the surface.
This slurry coating method has been applied to refractory metals and alloys for high temperature oxidation protection. It has also been used to produce a protective coating of carbides on a graphite surface. This research is described in the paper by H. S. Hu, A. Joshi and J. S. Lee entitled "Micro-structural Evaluations of a Si-Hf-Cr Fused Slurry Coating on Graphite for Oxidation Protection," published in the Journal of Vacuum Science & Technology, February 1991. The authors prepared a slurry with a powder mixture having the composition by weight percent: 60%Si-30%Hf-10%Cr. The carrier fluid was an organic lacquer vehicle, which was reportedly an organic binder in acetone. The authors stated that the coating was applied to a density of 14-16 mg/cm.sup.2 on a pyrolitic graphite surface, and then subjected to a vacuum fusion treatment at a temperature of 1450.degree. C., for a period of 45 minutes. The process was repeated to assure a complete and dense coating by applying a second coat. The complete coating was comprised of several layers containing various different metal carbides and silicon carbide. The authors noted some local oxidation of the graphite that was believed to be due to a crack in the coating, allowing oxygen to penetrate and oxidize the graphite. The oxidation resistance of the coating itself was attributed largely to the formation of silicon dioxide on the outermost surface layer.
This slurry coating method suffers from the same drawback as the CVD coating techniques discussed previously, namely that the buildup of the coating layers results in dimensional changes of the parts that are being treated. For high temperature gas flow systems with complex geometries, and for flow passages where the dimensions are critically important, it is desirable to overcome this drawback of the slurry coating technique.
In the slurry coating research mentioned above, it was found that the innermost layer of the coating was comprised largely of hafnium carbide. It was also reported that the free energy of formation at elevated temperatures is lower for hafnium carbide, and some chromium carbides, than for silicon carbide. Thus it is desirable to provide a method for treating surfaces of carbon-based materials, particularly graphite, to form protective layers of metal carbide, particularly HfC, without substantial changes in the dimensions of material or location of the surface, in a manner applicable to internal and external surfaces having complex geometry.